For main-group elements, as the atomic radius increases going down a group what else can be said:

• A. Electronegativity decreases.
• B. The mass of an element decreases. 
• C. The highest principal quantum number of the valence orbitals increases. 
• D. Choices A and C.
• E. All of the above.

Answer: (D)

Verified step by step guidance

1. Understand that atomic radius refers to the size of an atom, typically measured from the nucleus to the outer boundary of the electron cloud.
2. Recall that as you move down a group in the periodic table, atoms have more electron shells, meaning the principal quantum number (n) of the valence electrons increases.
3. Recognize that an increase in the principal quantum number means valence electrons are farther from the nucleus, which generally leads to a larger atomic radius.
4. Consider the concept of effective nuclear charge (Z_eff), which is the net positive charge experienced by valence electrons after accounting for shielding by inner electrons; this value does not significantly increase down a group and can even slightly decrease due to increased shielding.
5. Conclude that the primary reason atomic radius increases down a group is because the highest principal quantum number of the valence orbitals increases, placing valence electrons in shells farther from the nucleus, outweighing changes in effective nuclear charge.